The arithmetic sequence $(a_i)$ is defined by the formula: $a_i = -9 + 6(i - 1)$ What is $a_{15}$, the fifteenth term in the sequence?
Answer: From the given formula, we can see that the first term of the sequence is $-9$ and the common difference is $6$ To find $a_{15}$ , we can simply substitute $i = 15$ into the given formula. Therefore, the fifteenth term is equal to $a_{15} = -9 + 6 (15 - 1) = 75$.